A capacitor is a passive electronic component that is used to store energy in the form of an electrostatic field, and comprises a pair of electrodes separated by a dielectric layer. When a potential difference exists between the two electrodes, an electric field is present in the dielectric layer. An ideal capacitor is characterized by a single constant value of capacitance, which is a ratio of the electric charge on each electrode to the potential difference between them. For high voltage applications, much larger capacitors are necessary.
One important characteristic of a dielectric material is its breakdown field. This corresponds to the value of electric field strength at which the material suffers a catastrophic failure and conducts electricity between the electrodes. For most capacitor geometries, the electric field in the dielectric can be approximated by the voltage between the two electrodes divided by the spacing between the electrodes, which is usually the thickness of the dielectric layer. Since the thickness is usually constant, it is more common to refer to a breakdown voltage, rather than a breakdown field. There are a number of factors that can dramatically reduce the breakdown voltage. In particular, geometry of the conductive electrodes is important factor affecting breakdown voltage for capacitor applications. In particular, sharp edges or points hugely increase the electric field strength locally and can lead to a local breakdown. Once a local breakdown starts at any point, the breakdown will quickly “trace” through the dielectric layer until it reaches the opposite electrode and causes a short circuit.
Breakdown of the dielectric layer usually occurs as follows. The intensity of an electric field becomes high enough to “pull” electrons from atoms of the dielectric material and makes them conduct an electric current from one electrode to another. Presence of impurities in the dielectric or imperfections of the crystal structure can result in an avalanche breakdown as observed in semiconductor devices.
Another important characteristic of a dielectric material is its dielectric permittivity. Different types of dielectric materials are used for capacitors and include ceramics, polymer films, paper, and electrolytes of different kinds. The most widely used polymer film materials are polypropylene and polyester. Increasing the dielectric permittivity increases the possible volumetric energy density of a capacitor, which makes it an important technical task.
An ultra-high dielectric constant composite of polyaniline, PANI-DBSA/PAA, was synthesized using in situ polymerization of aniline in an aqueous dispersion of poly-acrylic acid (PAA) in the presence of dodecylbenzene sulfonate (DBSA) (see, Chao-Hsien Hoa et al., “High dielectric constant polyaniline/poly(acrylic acid) composites prepared by in situ polymerization”, Synthetic Metals 158 (2008), pp. 630-637). The water-soluble PAA served as a polymeric stabilizer, protecting the PANI particles from macroscopic aggregation. A very high dielectric constant of about 2.0×105 (at 1 kHz) was obtained for the composite containing 30% PANI by weight. Influence of the PANI content on the morphological, dielectric and electrical properties of the composites was investigated. Frequency dependence of dielectric permittivity, dielectric loss, loss tangent and electric modulus were analyzed in the frequency range from 0.5 kHz to 10 MHz. SEM micrograph revealed that composites with high PANI content (i.e., 20 wt. %) consisted of numerous nano-scale PANI particles that were evenly distributed within the PAA matrix. High dielectric constants were attributed to the sum of the small capacitors of the PANI particles. The drawback of this material is a possible occurrence of percolation and formation of at least one continuous electrically conductive channel under electric field with probability of such an event increasing with an increase of the electric field. When at least one continuous electrically conductive channel (track) through the neighboring conducting PANI particles is formed between electrodes of the capacitor, it decreases a breakdown voltage of such capacitor.
Colloidal polyaniline particles stabilized with a water-soluble polymer, poly(N-vinylpyrrolidone) [poly(′-vinylpyrrolidin-2-one)], have been prepared by dispersion polymerization. The average particle size, 241±50 nm, have been determined by dynamic light scattering (see, Jaroslav Stejskal and Irina Sapurina, “Polyaniline: Thin Films and Colloidal Dispersions (IUPAC Technical Report)”, Pure and Applied Chemistry, Vol. 77, No. 5, pp. 815-826 (2005).
Single crystals of doped aniline oligomers are produced via a simple solution-based self-assembly method (see, Yue Wang, et. al., “Morphological and Dimensional Control via Hierarchical Assembly of Doped Oligoaniline Single Crystals”, J. Am. Chem. Soc. 2012, 134, pp. 9251-9262). Detailed mechanistic studies reveal that crystals of different morphologies and dimensions can be produced by a “bottom-up” hierarchical assembly where structures such as one-dimensional (1-D) nanofibers can be aggregated into higher order architectures. A large variety of crystalline nanostructures, including 1-D nanofibers and nanowires, 2-D nanoribbons and nanosheets, 3-D nanoplates, stacked sheets, nanoflowers, porous networks, hollow spheres, and twisted coils, can be obtained by controlling the nucleation of the crystals and the non-covalent interactions between the doped oligomers. These nanoscale crystals exhibit enhanced conductivity compared to their bulk counterparts as well as interesting structure-property relationships such as shape-dependent crystallinity. Furthermore, the morphology and dimensions of these structures can be largely rationalized and predicted by monitoring molecule-solvent interactions via absorption studies. Using doped tetra-aniline as a model system, the results and strategies presented in this disclosure provide insight into the general scheme of shape and size control for organic materials.
Materials with high dielectric permittivity based on composite materials containing polarized particles (such as PANI particles) may demonstrate a percolation phenomenon. The layered polycrystalline structure formed by the composite material has multiple tangling chemical bonds on borders between crystallites. When the composite material that is used has a high dielectric permittivity and possesses a polycrystalline structure, a percolation may occur along the borders of crystal grains.
Hyper-electronic polarization of organic compounds is described in greater detail in Roger D. Hartman and Herbert A. Pohl, “Hyper-electronic Polarization in Macromolecular Solids”, Journal of Polymer Science: Part A-1 Vol. 6, pp. 1135-1152 (1968). Hyper-electronic polarization may be viewed as the electrical polarization of external fields due to the pliant interaction with charged pairs of excitons, in which the charges are molecularly separated and range over molecularly limited domains. Four polyacene quinone radical polymers were investigated. These polymers at 100 Hz had dielectric constants of 1800-2400, decreasing to about 58-100 at 100,000 Hz. An essential drawback of the described method of production of the material is the use of high pressure (up to 20 kbars) for forming the samples intended for measurement of dielectric constants.
The Influence on the dielectric properties of the acrylic acid grafted on to isotactic polypropylene was investigated using density functional theory calculations, both in the molecular modeling and three-dimensional (3D) bulk periodic system frameworks (see, Henna Russka et al., “A Density Functional Study on Dielectric Properties of Acrylic Acid Crafted Polypropylene”, The Journal of Chemical Physics, 134, 134904 (2011)). In the molecular modeling calculation which modeled, the polarizability volume, and the polarizability volume per mass, which reflects the permittivity of the polymer, as well as the HOMO-LUMO gap, it was discovered that one of the important measures indicating the electrical breakdown voltage strength was chain length and carboxyl mixture ratios.
The lowest unoccupied molecular orbital (LUMO) energies of a variety of molecular organic semiconductors have been evaluated using inverse photoelectron spectroscopy data and are compared with data determined from the optical energy gap, electrochemical reduction potentials, and density functional theory calculations (see, Peter I. Djuravich et al., “Measurement of the lowest unoccupied molecular orbital energies of molecular organic semiconductors”, Organic Electronics, 10, pp. 515-520, (2009)).
Capacitors as energy storage devices have well-known advantages versus electrochemical energy storage, e.g. a battery. Compared to batteries, capacitors are able to store energy with very high power density, i.e. charge/recharge rates, have long shelf lives with little degradation, and can be charged and discharged (cycled) hundreds of thousands or millions of times with little degradation. However, capacitors often do not store energy in small volume or weight as in case of a battery, or at low energy storage cost, which makes capacitors impractical for some applications, for example electric vehicles. Accordingly, it may be an advance in energy storage technology to provide capacitors of higher volumetric and mass energy storage density and lower cost.